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OM2500A Power Amplifier | Radioaficion Ham Radio

OM2500A Power Amplifier


OM Power

External Radio Frequency Power Amplifier OM2500A

Power Amplifier OM2500A

The OM Power model OM2500A is a fully automatic, full legal limit amplifier, designed for heavy duty use on all short wave amateur bands from 1.8 to 29.7 MHz (including WARC bands) and all modes with no time limit. It is equipped with one GU84b ceramic tetrode.


  • Full compatibility with: ICOM, ELECRAFT,KENWOOD, TEN-TEC ORION,
  • Yaesu and CI-V data.
  • Automatic switching between bands
  • Automatic tuning with the band according to segments
  • Automatic switching of band pass filter
  • Automatic switching of antenna switches
  • The possibility to use 2 different antennas within one band or segment



Frequency Coverage:......... Amateur Bands 1.8 - 29.7 MHz including WARC

Power Output:................... 1500w PEP continuous output all modes - *no time limit*

Input Power:...................... 60 - 80W for full output power

Input Impedance:................ 50 Ohm, VSWR < 1.2 : 1

Power Gain:..........................15 dB

Output impedance:................ 50 Ohm unbalanced

Maximum output SWR:............2:1

SWR protection:.................automatic switching to STBY , when reflected power is 350Wor higher

Intermodulation distortion: ........36 dB below nominal output

Suppression of harmonics:......... < -52 dBc second, < -65 dBc third

Tube:..................................... GU84b Ceramic tetrode

Cooler:.................................... Centrifugal blower + axial blower

Power supply:............................ 240 V - 60 Hz

Transformers:.......................... 2 pcs of toroidal transformer 2,0 kVA

Parameters:.............485 mm x 200 mm x 455 mm [19.1" x 7.9" x 17.9"] (width x height x depth)

Weight:................... 41,5 kg ( 92 lb )

Protection Circuits

There are 8 special protection circuits used in the amplifier. They are activated when one or more of next parameters exceed defined values or some unwanted occasion occurs.

  • VSWR too high
  • Anode current too high
  • Screen current too high
  • Grid current too high
  • Mistuning of PA
  • Hot switching protection
  • Soft start for protecting your fuses
  • "switch-on blocking " at opened amplifier


There are couples of LED and bar graph indicators visible on the front panel to inform you about value of some parameters or operation status:

Bar graph indicators

  • Power output - 50 LED
  • Reflected power - 20 LED
  • Current at screen Ig2 - 10 LED
  • Anode voltage, anode current, tuning - 30 LED

LED Indicators

  • Current at control grid (Ig1 - 2 LED)
  • WAIT - preheating of tube (180 sec)
  • STBY - standby
  • OPR - operation condition
  • FAULT - failure, switching off for abt. 4 sec
  • INHIBIT - operating condition
  • ANT 1 - which ANT is use
  • ANT 2 - which ANT is use
  • ANT - ANT selection


  • UP - DWN - listing in menu
  • MAN - manual operating
  • AUTO - automatic operating
  • TUNE - tuning
  • SET - confirms selected parameter

OSD Indicator

  • LCD Display 2 x 16 characters

HF part

In this amplifier a tetrode GU 84b is used in a grounded-cathode circuit (input into control grid). This amplifier achieves excellent linearity by the voltage stabilization of the control grid bias and the screen voltage. The power input is given to the control grid, using a broadband input circuit with an input impedance of 50 Ohms. This adaptable input circuitry ensures a good input SWR (better than 1.5:1) on all amateur bands.

The output of the amplifier is a Pi-L circuit. The ceramic capacitor for TUNE and LOAD are divided. This enables the amplifier to be tuned exactly and makes it possible to easily return to the previously set positions after band changes.

Top view on the opened OM2500A

 Power Amplifier OM2500A Top view on the opened OM2500A

Power Supply

Power supply of the amplifier is comprised of two of 2,0 kVA toroidal transformers. A soft start is provided using relays and resistors.

The high anode voltage is made by combining 8 x 350 V (total 2800V) @ 2A. Each has its own rectifier and filter. In the high voltage circuit, safety resistors are employed to protect the amplifier against overload.

The source for screen grid is regulated by parallel stabilization with BU508 transistors and delivers a voltage of 360V at 100mA.

The -120V for the control grid is regulated with zener diodes.

Rear view of the amplifier OM2500A

Rear view of the amplifier OM2500A

I/O Box and Interface

Control of Amplifier and communication with TCVR as well as Antennas / BPF switching can be done via the rear panel I/O Interface.

Control cable maintains TX / RX switching of the PA (TX GND). The cable is shielded. On the side of the power amplifier a CINCH-socket is used. On the side of your transceiver you have to use a socket suitable for this transceiver. During transmitting the middle pin is connected to the ground.

The relays of the OM2500A have to be switched earlier than HF is applied (cold switching). Modern transceivers they have a time delay between PTT switching and power output.

The amplifier is equipped with two safety devices, which ensure that the output relay is not switched under power mistakenly (hot switching).

Power Amplifier OM2500A

KEY IN RCA Phono - Input signal PTT switching voltage / current 5V /2 mA)

KEY OUT RCA Phono - Output signal PTT (maximum switching of 30V / 50mA)

CI-V Mono 3.5mm Jack for connection of ICOM TCVRs or devices that provide compatible CI-V protocol. Correct baud rate is important.

TCVR DB9 serial port RS232 for KENWOOD, YAESU and ELECRAFT TCVRs. Correct baud rate and type is required for successful operation. If both CI-V and TCVR cables are connected then CI-V disables RS232. Otherwise selection of interface is done via TCVR type.

PC DB-9 RS232 port is used for communication with your PC. Please use setting you would normally use if using direct TCVR - PC connection.

ALC RCA Phono - Automatic Level Control is used when tuning the PA to block drive level .

WARNING! We only recommend using ALC feature while operating RTTY, FM and other 100% duty modes.


The CONTROL socket is a single DB-15 connector that provides many connections to the amplifier from your transceiver. Use shielded cable for all connections to this connector. You will need to fabricate a cable with the proper connector for your transceiver or use individual connectors as described below.

1.  ALC Out
2.  NC
3.  INHIBIT Control voltage
4.  TX INHIBIT for Yaesu and Elecraft – this supersedes
ALC output
5.  NC
7.  NC
8.  KEY IN
9.  –  
15.  GND


DB-25 is used for switching of external HP BPF or external Antenna Switch. Maximum switching of 30V / 0.5A is possible.

1.  antenna port 1   
2.  antenna port 2
3.  antenna port 3
4.  antenna port 4
5.  antenna port 5
6.  antenna port 6
7.  antenna port 7
8.  antenna port 8
9.  antenna port 9
10. antenna port 10
11. COMMON port of ANT SW  
12. NC
13. GND
14. BPF 160m   
15. BPF 80m
16. BPF 40m
17. BPF 30m
18. BPF 20m
19. BPF 17m
20. BPF 15m
21. BPF 12m
22. BPF 10m
23. COMMON BPF port
24. NC
25. GND

OM Power Amplifier OM2500A RESOURCES

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